Forward differences Jacobian assembler

An implementation of forward differences numerical Jacobian, similar to the existing central differences numerical Jacobian computation.

It takes less time to compute the Jacobian and shows better overall performance despite the reduced accuracy of the Jacobian.

The unit tests are same as those for the central differences Jacobian but with reduced error tolerance. Some refactorings were done trying to reduce duplicated test code. Unfortunately, the test structure is on the monolithic side and my separation efforts didn't succeed.

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